1. Field of the Invention
The present invention relates to method for injection-molding a plastic substrate used in high-density information recording carriers or media such as optical disks, optical cards, optical tape or the like. More particularly, it relates to an improvement in an injection-molding process for molding a transparent plastic substrate for DRAW type and Magneto-Optical type recording disks.
2. Description of the Related Art
In the high-density information recording carriers or media such as magneto-optical recording disks in which information is recorded in a form of spots each having a diameter sub-micron order by a laser beam which passes through the plastic substrate and is focused on a recording layer supported on the plastic substrate, it is requested to reduce the birefringence in the transparent plastic substrates.
In the case of the magneto-optical recording system, a very small change of polarized light such as 0/1 to 0.3 degree must be detected and hence the transparent plastic substrates having high birefringence which deteriorate CN ratio (carrier to noise ratio) are not applicable to practical uses.
The transparent plastic substrate is advantageously produced from injection-molded polycarbonate resin because of low cost and low moisture absorbing property thereof. The substrate made of polycarbonate resin, however, shows rather high birefringence values.
In order to lower the birefringence of polycarbonate substrates, injection-compression molding technique is usually used (Japanese patent laid-open No. 62-9,926). Although this technique is very effective to reduce the birefringence, the conventional injection-compression molding technique has such a problem that a fine concave-convex pattern consisting of formatting pits and/or trucking guide grooves engraved on a surface of a stamper by a laser cutting technique is difficult to be transferred precisely onto a surface of the plastic substrate.
Generally, the improvement in the pattern transferring property and the improvement in birefringence conflict with each other.
Still more, in the case of plastic substrates having a large diameter of about 300 mm produced by the conventional injection-compression molding process, the dimensional stability of the molded disks is poor, so that they are apt to twist or bend.
Therefore, it is not easy to produce a transparent plastic substrate which satisfies the three requirements of low birefringence, high pattern transferring property and high dimensional stability simultaneously.
The present applicant proposed improved injection-molding processes for lowering remarkably the birefringence in co-pending Japanese patent application Nos. 61-45,900 and 61-80,189. The present invention was completed in the same line as these patent applications.
An object of the present invention is to provide an improved process for injection-molding a plastic substrate, particularly a substrate made of polycarbonate resin, used in high-density information recording media.